Flexography is a method of direct rotary printing that uses resilient relief plates of rubber or photopolymer material. The plates may be flat or in the form of sleeves and in both cases they are used in the printing process by securing them to a cylinder where they are inked by a cell-structured Anilox metering roller. Flexographic presses can be used to print on a large range of substrates and are particularly used in the packaging industry, for printing on board, paper, foil and film.
Compared to other printing processes such as offset lithography or gravure, the relief image on the flexographic plate is very thick—mostly of the order of millimeters high. A popular method of producing such plates is by using thick photopolymerizable layers and imaging by hardening through a photo-tool. The unhardened photopolymer precursor may then be washed away with solvent. This process requires the pre-imaging of the photo-tool which is usually in the form of a silver halide based film and then multiple steps to prepare the flexo plate by UV exposure, washing, drying, etc. A description of the flexographic process including plate-making may be found in “The Printing Ink Manual”, edited by R. H. Leach and R. J. Pierce—5th edition, published in 1993 by Blueprint Press (pages 33 to 42).
Process simplification and acceleration of plate-making are desirable targets for flexography in the same way as they have been for offset lithography. Thus, the computer-to-plate concept which has become popular for offset has also been exploited for flexography. For instance, U.S. Pat. No. 5,654,125 describes the formation of an integral photo-tool in a process and apparatus for image-wise ablating an infrared radiation sensitive layer of a flexographic element, for use in making a flexographic printing plate. The photopolymer flexo plate has a top coating, which may contain a high concentration of carbon black. The layer absorbs infrared radiation and when imaged using an infrared emitting laser as an imaging source, areas become hot from infra red absorption and are ablated away in what will be the image areas. The layer in which selected areas have been ablated becomes the integral photo-tool and UV exposure is made through these ablated gaps in the coating. Where the carbon layer is unablated it prevents the photopolymerization of the UV sensitive layer. Where the carbon has been ablated away the UV light causes polymerization of the photopolymer. The plate is then washed to remove the carbon layer and the unpolymerized photopolymer precursor. Thus, such plates can be considered as digital and are analogous to the computer-to-plate technology developed for offset lithographic printing. The process eliminates the need for the silver halide photo-tool, which involves dark room facilities as well as the use of unstable environmentally undesirable processing liquids. The plates give enhanced quality over conventional plates, but require relatively high energies (up to 5joules per square cm.) to ablate the carbon layer. This is because the layer needs to be thick enough to give a high optical density to block the UV exposure needed to polymerize the relatively thick pre-polymer layer. Such plates are relatively expensive and still involve a multiplicity of stages in their preparation.
Alternative methods of using digital imaging in the production of relief printing plates are based on inkjet. The use of ink jet as a means for producing relief images is suggested in UK Pat. No. 1431462. Here, the inkjet fluid alters the solubility properties of the coating on a substrate. The image produced is only a few microns thick and can be used for offset lithography. However, the imaged plate is not of a flexographic nature, since the surface does not have suitably elastomeric properties.
An alternative method of digital plate formation is to ink jet the mask onto the plate and a number of inventions have been published using this idea. Here, the background must be covered with the masking ink and the unimaged areas from the inkjet part of the process are then used as the final relief areas of the printing plate.
U.S. Pat. No. 5,779,779 uses a Hot Melt inkjet ink for UV blocking on the flexo plate. U.S. Pat. No. 6,358,668 uses a water-based inkjet ink and a special ink-receiving layer coated onto the flexo plate to ensure good UV blocking properties for the ink. The use of ink jet inks as masks still requires subsequent wet stages of washing out uncured material and then drying the plate.
U.S. Pat. No. 5,511,477 describes the formation of various types of printing plates including flexographic printing plates using a UV curable ink jet ink, by directly jetting onto a substrate base. However, the actual height of the image is only that of one layer of the inkjet drops and thus it would be difficult to produce flexographic plates of acceptable performance, because flexography is designed for plate image thickness in the order of magnitude of millimeters. Ink jet produces images in the order of magnitude of microns.